Hello!

This is my fourth summer growing dahlias and I have encountered something strange that has not happened to me before. I started 20 tubers in pots indoors. Once I moved them outside, 4 out of the 20 just flopped over at the base of the stem. I dug the tubers up to see what happened, and the stem right at the soil had just shriveled up. I am so sad! Could it be too much water? Too much fertilizer? I used a Scott's planting mix that already contains fertilizer, but nothing additional. The tuber and the stem an inch above the tuber were fine. It was just that inch or so of stem at the dirt line that got weak. Has this happened to anyone else? Any advice would be greatly appreciated!!

Thanks!